Symbol referencing errors

Question

I'm getting this error when I compile with g++ thread.cpp -o thread -lpthread and I can't seem to find the referencing error:

Undefined                       first referenced
symbol                             in file
sem_destroy                         /var/tmp//ccfHWR7G.o
sem_init                            /var/tmp//ccfHWR7G.o
sem_post                            /var/tmp//ccfHWR7G.o
sem_wait                            /var/tmp//ccfHWR7G.o
ld: fatal: symbol referencing errors. No output written to thread
collect2: ld returned 1 exit status

My header file just contains some global variables and function names:

#ifndef THREAD_H_
#define THREAD_H_

#define NUM_THREADS 6

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <iostream>
#include <semaphore.h>
#include <fstream>
#include <unistd.h>

using namespace std;

sem_t SEM;
fstream of;
pthread_t thread[NUM_THREADS];
pthread_attr_t attr;
int rc;
long t;
void *status;
void *Busy_Work(void *t);
void Open_Initialize_File();
void Initialize_Set_Attribute();
void Create_Thread();
void Free_Attricutes_Wait();




#endif /* THREAD_H_ */

This is my main file and check with an IDE there wasn't any basic syntax that I missed:

#include "thread.h"

int main (int argc, char *argv[]) {
Open_Initialize_File();
Initialize_Set_Attribute();
Create_Thread();
Free_Attricutes_Wait();
sem_destroy(&SEM);
cout << "Main: program completed" << endl << "Exiting" << endl;
pthread_exit(NULL);
}

void Open_Initialize_File() {
of.open("SHARED.txt");
of << pthread_self() << "\r" << endl;
of.close();
}

void Initialize_Set_Attribute() {
sem_init(&SEM, 0, 1);
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_JOINABLE);
}

void Create_Thread() {
for (int t = 0; t < NUM_THREADS; t++) {
    rc = pthread_create(&thread[t], &attr, Busy_Work, (void *)(t+1));
    if (rc) {
        cout << "ERROR: return code from pthread_create() is " << rc << endl;
        exit(-1);
    }
}
}

void *Busy_Work(void *t) {
int i;
long tid = (long)t;
for (i=0; i<10; i++)
{
    sem_wait(&SEM);
    cout << "Thread" << tid << "Thread id< " << pthread_self() << " >running..." << endl;
    of.open("SHARED.txt",std::fstream::app);//opening file in append mode
    of << pthread_self() << "\r" << endl;
    of.close();
    sem_post(&SEM);
    if(tid%2==0)
        sleep(2);
    else
        sleep(3);
}
pthread_exit((void*) t);
}

void Free_Attricutes_Wait() {
pthread_attr_destroy(&attr);
for (t = 0; t < NUM_THREADS; t++) {
    rc = pthread_join(thread[t], &status);
    if (rc) {
        cout << "ERROR: return code from pthread_join() is" << rc << endl;
        exit(-1);
    }
    cout << "Thread " << t << " Thread id <" << pthread_self() << ">exited" << endl;
}
}

The error means that the linker cannot find the indicated symbols. You probably need to link to some additional library, like rt (-lrt). What platform are you working on? — Martijn Wijns, Jun 05 '14 at 19:57
check if the file stream you open is ok, I think you also need to specify std::fstream::out. — Martijn Wijns, Jun 05 '14 at 20:13
slightly off topic, but *please* don't `use namespace std;`! — Rob K, Jun 05 '14 at 20:42
possible duplicate of [What is an undefined reference/unresolved external symbol error and how do I fix it?](http://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix) — alk, Jun 08 '14 at 15:20

score 3 · Accepted Answer · answered Jun 05 '14 at 20:38

The linker is missing some symbols, that might indicate that some additional external library needs to be linked as well. On a unix platform a good way to figure out how to fix this is to have a look at the man pages:

man sem_init

This will usually tell you what you need to do. In this case it would specify that you need to link with -lrt

Symbol referencing errors

1 Answers1